For centuries, barrier systems of various types have been employed to impede or prevent the infiltration of undesirables into an area. From the earliest moats and high castle walls, to present day mine fields, barrier systems have provided impediments to infiltration.
Yet while such historic barrier systems have achieved successes in preventing the infiltration of adversaries, they unfortunately do not discriminate and consequently may serve to inhibit both friend and foe alike. In addition, certain barrier systems such as mine fields remain active indefinitely, or at least until they are removed and/or deactivated—a task that is both dangerous and prohibitively time consuming.
Consequently, barrier systems that inhibit foes while permitting friends and which do not require dangerous deactivation and/or removal would represent a significant advance in the art.